DuckDB reserved keywords used as column/field names cause parse errors in pushed-down queries

[sfc-gh-okalaci]

I realized while reviewing #274, but seems a lot more common than that.

CREATE TABLE t_pivot_bug (pivot int, b text) USING iceberg;
CREATE TABLE
Time: 21.265 ms
postgres=# INSERT INTO t_pivot_bug VALUES (1, 'hello');
ERROR:  Parser Error: syntax error at or near "pivot"

LINE 1: COPY (SELECT pivot, b FROM read_csv('/Users/okalaci/pg-install/data_pg17...
                     ^

LINE 1: COPY (SELECT pivot, b FROM read_csv('/Users/okalaci/pg-install/data_pg17...
                     ^
Time: 8.201 ms

quote_identifier() uses PostgreSQL's keyword list to decide whether to double-quote identifiers in SQL sent to DuckDB (pgduck_server). Identifiers that are DuckDB RESERVED_KEYWORD but not PostgreSQL keywords pass through unquoted, causing DuckDB parse errors.

This affects:

1. Column names in any pushed-down SELECT or INSERT...SELECT (deparse layer in deparse.c)
2. Composite field names in the temporal validation struct_pack generation (iceberg_query_validation.c)

Affected keywords

DuckDB RESERVED_KEYWORD tokens that are not PostgreSQL keywords (and thus never quoted by quote_identifier()):

Keyword	DuckDB category	PostgreSQL status
describe	RESERVED	not a keyword
lambda	RESERVED	not a keyword
pivot	RESERVED	not a keyword
pivot_longer	RESERVED	not a keyword
pivot_wider	RESERVED	not a keyword
qualify	RESERVED	not a keyword
summarize	RESERVED	not a keyword
unpivot	RESERVED	not a keyword
show	RESERVED	unreserved (not quoted)

Source: duckdb_pglake/duckdb/third_party/libpg_query/include/parser/kwlist.hpp

[sfc-gh-okalaci]

We should perhaps introduce duckdb_quote_identifier function that quotes these ?

[sfc-gh-okalaci]

Current workaround is to change such column names to names that Postgres quotes

CREATE TABLE t_pivot_bug ("Pivot" int, b text) USING iceberg;
CREATE TABLE
Time: 41.900 ms
postgres=# INSERT INTO t_pivot_bug VALUES (1, 'hello');
INSERT 0 1
Time: 83.137 ms

[sfc-gh-dachristensen]

Seems like a pretty easy generated-at-build-time function just given that keyword table; presumably we check for one of the reserved keywords and if found quote it, otherwise returning quote_identifier().

[sfc-gh-dachristensen]

#297 should take care of this one. (Also added some additional quoting-related fixes for STRUCT parsing.)